We have developed a device to splice plastic optical fibers by thermal fusion. The device heats the joint of fibers inserted in a PEEK tube, which can help align the fiber axes and protects the fused joint mechanically. We have tuned the device to produce splices with good light transmission and mechanical strength. Light transmission of nearly 95% with an rms variation of 1.5% has been achieved. The joint supports 1.2 kgf in tension and the bending radius can be as small as 3 cm. The mechanical strength can be improved easily by adding epoxy. We describe the optimization of the device and measurement results of 28 000 fiber splices for the CDF plug-upgrade tile/fiber calorimeter.